1. Field of the Invention
The present invention relates to lightweight high-stiffness composite panels used in aircraft, motor vehicles, vessels, buildings, or general industrial equipment. More particularly, it is concerned with methods of forming composite panels having a honeycomb sandwich structure by using an RTM (Resin Transfer Molding) process.
2. Description of the Related Art
Previously known methods of forming honeycomb sandwich panels include a molding method in which outside plates previously formed as skins are placed on top and bottom surfaces of a honeycomb core with an adhesive applied between them and the outside plates and the honeycomb core are bonded together under heat and pressure, as well as a molding method in which prepreg sheets and a honeycomb core are stacked by hand lay-up operation with an adhesive applied between them, the prepreg sheets and honeycomb core thus stacked are covered with a vacuum bag, an internal space of the vacuum bag is drawn by vacuum, and then the stack is subjected to heat and pressure by using a hot press or an autoclave.
In addition, Japanese Patent Laid-Open No. 295362/1997 describes a method of forming a honeycomb sandwich panel by the RTM process, in which an impregnating resin is kept from flowing into a honeycomb core by use of a resin/moisture barrier film formed of a particular macromolecular film.
Since the aforementioned method of forming honeycomb sandwich panels utilizes prepreg as a raw material, it requires high material cost and expensive facilities for the storage of materials and for a curing operation. Further, this method involves many processing steps, and this would develop a problem of cost reduction of composite materials.
In the aforementioned RTM process for forming a honeycomb sandwich panel by use of a particular macromolecular film, it is necessary to apply an adhesive to both sides of the macromolecular film because outside plates and a honeycomb core are bonded with the macromolecular film (thermoplastic resin) placed between them. As a method of forming alternative panels for existing sandwich panels, the RTM process would pose new problems, such as an increase in weight due to the use of additional material.
Furthermore, when a panel of a complex shape having a steplike portion or a curved surface, for instance, is formed by the aforementioned method of forming the honeycomb sandwich panel that uses the macromolecular film, it is difficult for a finished panel to keep its shape and the desired shape will not easily be obtained due to poor adhesion and low flexibility of the macromolecular film. Moreover, sealing performance against resin would become insufficient and the stiffness of the finished panel would deteriorate.